The invention relates to rotary drill bits and a method of use and in particular to such bits which are used to drill holes in subsurface formations to extract oil, gas or water or in mining or in the removal of cores.
The drill bits comprise a bit body having a passageway within the body, e.g. for a fluid such as a drilling mud, opening at an external surface of the body, elements being mounted on the external surface of the body and many fluid channels extending from the passageway opening and past some of the elements. The elements may be formed of diamond, synthetic diamonds or the like and they may cut into the formation by a true cutting action or by an abrading action. Such bits are disclosed in, for example, U.S. Pat. Nos. 2,371,489, 2,809,808, 3,709,308 and 3,727,704. In use of such a drill bit, drilling fluid is pumped through the fluid passage within the bit body and emerges through the opening and flows upwardly along the many fluid channels. The fluid flushes cuttings away from the drill bit and cleans and cools the cutting or abrading elements, and to a certain extent cools the formation being drilled.
In known drill bits of this type the opening or openings are usually located in the region of the central axis of the bit, and there are many fluid channels which extend away from the centre of the bit. There must be sufficient fluid channels or waterways to ensure that cuttings are removed from all parts of the bit and depending on the design there can be as many as 30 waterways. The channels are often substantially straight as viewed axially of the bit, but they may also have a circumferential component so as to impart a somewhat spiral flow to the fluid as it flows away from the opening or openings in the region of the central axis of the bit.
In all drill bits of this type there is a tendency for the channels to be blocked by cuttings removed from the formation, and where there are many channels, blockage of one channel means that its associated cutting elements are not cooled and cleaned and the remaining channels have to cope with the entire flow of fluid. Although there will result some increase in pressure in the channels which remain unblocked, this increase in pressure will not generally be sufficient to unblock the blocked channel, so that the cutters associated with that channel will become substantially ineffective through overheating and clogging, which presents a barrier between the formation and the cutting element. This problem is more pronounced when the drill bit is used with a water-based mud, which has a greater tendency than oul-based invert emulsion mud to allow the cuttings to block the drilling fluid channels. There is a great risk of a blockage when drilling in a plastic formation, e.g., claystone, shale.
According to one aspect of the present invention, there is provided a rotary drill bit for use in subsurface formations comprising a bit body, a passageway within the body, e.g. for a drilling fluid such as a mud, opening at an external surface of the body, elements mounted on the external surface of the body for cutting or abrading the formation, a plurality fluid channels extending from the passageway opening and past some of the elements, characterised in fluid unbranched channels, which are arranged to cause the fluid to flow past a plurality of the elements.
Our investigations have shown that where the drilling fluid is arranged to flow in only one path along a fluid channel there are surprising advantages. If a blockage occurs in the channel the resulting constriction will cause the fluid pressure upstream of the blockage to rise substantially and this will tend to break down a partial or full blockage and so clear it.
Most preferably one elongate channel is present and extends in a spiral about the bit in the region of the cutting elements. There may also be two generally parallel channels each arranged in a helix and extending away from the passageway opening, preferably on diametrically opposite sides thereof. When a blockage occurs in the case of a channel arranged in a spiral, in a convolution there will be a substantial rise in fluid pressure on the upstream side of the blockage. This convolution will be closely within the convolution on the immediate downstream side of the blockage, so that there will be a large pressure difference across the land between the two convolutions. The fluid will tend to flow from the upstream convolution into the downstream convolution, due to this pressure difference, thus effectively by-passing the blockage and ensuring that cutting elements downstream of the blockage are still adequately cooled and cleaned.
Sometimes the downstream end of said spiral channel leads into an annular channel encircling the bit body and where the bit includes a gauge portion the annular channel may encircle the bit body adjacent the gauge portion.
In one preferred embodiment the fluid channel is of approximate uniform corss-sectional shape over most of its length. The cutting elements which are most preferably "preforms" are located in the channel in the floor or sidewalls thereof, advantageously being set into the floor or walls in such a way as not to interrupt the fluid flow.
The invention further includes a method of drilling a hole in an underground formation by means of a rotary drill bit and passing drilling mud through the bit to wash away cuttings, characterised in that the bit used is according to this invention and in that drilling fluid is passed unidirectionally along one or both of the one or two fluid channels to remove the cuttings.
Water-based muds are often preferred compared to oil-based muds and use of a bit of the invention reduces the risk of blockages when using such muds.